1. Field of the Invention
This application relates to the field of medical devices and more particularly to the field of coatings for medical devices.
2. Description of Related Art
Arteriosclerosis is a condition that detrimentally affects many individuals. Untreated, arteriosclerosis may lead to severe consequences, including heart damage, heart attack and death. Known treatments for arteriosclerosis have had limited success.
Transluminal balloon angioplasty, wherein a balloon is inserted via a catheter into the artery of the patient and expanded, thereby simultaneously expanding the partially closed artery to a more open state, is a well-known treatment for arteriosclerosis, but long-term benefits of balloon angioplasty are limited by the problems of occlusion and restenosis, which result in re-closure of the artery.
A variety of intravascular stents and prostheses have been developed to support diseased arteries and thereby inhibit arterial closure after angioplasty. In particular, expandable intraluminal stents have been developed in which a catheter is used to implant a stent into the artery of the patient in a minimally invasive manner.
Like other foreign bodies placed into arteries, stents can result in coagulation or thrombosis in the intravascular environment. Thrombosis can inhibit blood flow through the stent, diminishing its effectiveness, or can cause clotting, which can threaten the life of the patient. Accordingly, methods of reducing thrombotic activity have been sought to reduce the negative side effects caused by certain stents.
A number of coatings have been developed for medical devices that are intended to promote compatibility between a particular medical device and the environment in which the medical device resides. Some of these coatings, known as thromboresistant coatings, are intended to reduce the thrombosis often associated with insertion of a foreign object, such as a medical device, into the interior of the body.
Heparin, or heparinic acid, arteven, or leparan, is a glycosaminoglycan with well-known anticoagulant activity. Heparin is biosynthesized and stored in mast cells of various animal tissues, particularly the liver, lung and gut. Heparin is known to have antithrombotic activity as a result of its ability to bind and activate antithrombin III, a plasma protein which inhibits several enzymes in the coagulation cascade. It has been hoped that heparin coatings, by inhibiting thrombogenesis, can improve the therapeutic outcomes derived from intra-vascular medical devices, such as stents.
However, known heparin coatings are subject to a number of defects, including incompatibility with the organism and/or microscopic features of the surface to be coated, excessive thickness, difficulty in application, and insufficient durability. For example, several known coatings are based upon simultaneous coulombic interactions between heparin and tri(dodecyl)methylammonium chloride, which is also referred to herein as heparin-TDMAC, and hydrophobic interactions between the quaternary ammonium ion of heparin-TDMAC and the surface of the device. Due to the relative weakness of hydrophobic interactions, such coatings typically leach away from the substrate to which they are applied within a few hours; coatings of this type, therefore, are not generally durable enough to provide beneficial therapeutic results.
Other known coatings comprise silanes having a pendent amino or vinyl functionality. In the fabrication of these coatings, a base layer of silane is applied initially to the surface, followed by the application to the base layer of a second layer comprising antithrombogenic biomolecules, such as heparin. It is necessary that the pendent groups of the base layer of silane be both complementary and accessible to groups on heparin. In some such coatings, a silane with terminal amino functionality is applied to a substrate to form a first layer, followed by application of heparin in solution to form the second layer. In certain examples of this strategy, the amino functionality of the silane base layer reacts with an aldehyde-containing heparin derivative to form a Schiff base and thereby covalently attach the biomolecule to the base layer. In another group of coatings of this general class, a base layer comprising a silane with a vinyl functional group is applied to a surface, followed by covalent attachment, via free radical chemistry, of a heparin-containing derivative to the base layer.
Some of the known coatings have been found lacking in bioeffectiveness and stability. Modifications made in these coatings utilize additional coatings of polymeric matrices comprising reactive functionalities. The multi-step process required to fabricate the polymeric matrices necessary in these approaches increases the thickness of the resulting coatings. Thick coatings present a number of difficulties. First, thick coatings increase the profile of the medical device in the intravascular environment. A stent with a thick profile, for example, can reduce blood flow, thereby undermining the therapeutic benefit of the stent. A thick coating may also render the coating itself more vulnerable to pitting, chipping, cracking, or peeling when the stent is flexed, crimped, expanded, or subjected to intravascular forces. Any of the foregoing results of excessively thick coatings may reduce the antithrombogenic characteristics of the stent. Moreover, the likelihood of pitting is hypothesized to be greater in thick coatings, and pits in a coating may increase the susceptibility to galvanic corrosion of the underlying surface. Because their fabrication requires additional steps, coatings comprising multiple layers may also be more difficult and expensive to manufacture.
Accordingly, a need exists for a thromboresistant coating that is thin, durable, and biocompatible, and that may be applied in a single coating.